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The Problem
In the autumn of 1919, Popper struggled with a major problem. He asked "When should a theory be ranked as scientific?" and "Is there a criterion for the scientific character or status of a theory?" (Popper, 1963). In other words, what made a theory true science versus pseudoscience? -
Dissatisfaction
In the summer of 1919, Popper began to feel dissatisfied with the Marxist theory of history, psychoanalysis, and individual psychology (Popper, 1963). This dissatisfaction stemmed from their "claims to scientific status," which Popper did not agree with (1963). -
Conclusions
During this time, Popper made some very important conclusions about theories and the validity of their scientific status. These conclusions could also be considered criteria for scientific theories. Among these conclusions were the dismissal of irrefutable theories as being scientific (making these pseudoscience) and the idea of falsifiability. -
Falsification
It was in this year that Popper published his work "Conjectures and Refutations". It was in this publication that Popper explains his criteria of falsification for what makes a theory scientific. Falsification is the idea that a theory that is irrefutable is not scientific. In order for a theory to be scientific, it must be falsifiable and therefor testable. This is Popper's greatest contribution to the philosophy of science. Crash Course Video: https://www.youtube.com/watch?v=-X8Xfl0JdTQ